Cascade diffuser having thin, straight vanes

ABSTRACT

A cascade diffuser providing high pressure recovery in radial flow turbomachinery has first and second vaned diffuser stages arranged between an impeller and collector. The first and second stages are formed by thin, straight vanes which contribute substantially to the pressure recovery rate. The first stage vanes are preferably spaced apart from the impeller to provide a vaneless region for initially conditioning fluid flow from the impeller. The leading edges of the first stage vanes are preferably tapered to provide a semi-vaneless transition region between the vaneless region and first diffuser stage. The diffuser walls form a flow path between the impeller and collector between which the first and second stage vanes are disposed.

United States Patent 9] Dean, Jr.

[ Jan. 21, 1975 CASCADE DIFFUSER HAVING THIN,

STRAIGHT VANES [52] US. Cl. 415/211, 415/207 [51] Int. Cl. F04d 17/08,F04d 29/44 [58] Field of Search ..415/211,181, 207,163

[56] References Cited UNITED STATES PATENTS 1,187,428 6/1916 Homershan415/163 2,111,136. 3/1938 Bauer ...415/211 3,069,070 12/1962 Macaluso etal. 415/211 3,184,152 5/1965 Bourguard 415/211 3,424,372 1/1969 Blattneret al. 415/211 3,442,441 5/1969 Dettmering 415/181 FOREIGN PATENTS ORAPPLICATIONS 317,623 1/1957 Switzerland 415/211 971,224 7/1950. France415/211 Primary Examiner-Henry F. Raduazo Attorney, Agent, orFirm-Phillips, Moore, Weissenberger, Lempio & Strabala [57] ABSTRACT Acascade diffuser providing high pressure recovery in radial flowturbomachinery has first and second vaned diffuser stages arrangedbetween an impeller and collector. The first and second stages areformed by thin, straight vanes which contribute substantially to thepressure recovery rate. The first stage vanes are preferably spacedapart from the impeller to provide a vaneless region for initiallyconditioning fluid flow from the impeller. The leading edges of thefirst stage vanes are preferably tapered to provide a semivanelesstransition region between the vaneless region and first diffuser stage.The diffuser walls form a flow path between the impeller and collectorbetween which the first and second stage vanes are disposed.

1 Claim, 11 Drawing Figures sum NF 4 6 PATENIEB JANZI I975 CASCADEDIFFUSER HAVING THIN, STRAIGHT VANES BACKGROUND OF THE INVENTION Thepresent invention relates to radial flow diffusers for arrangementbetween an impeller and a collector of radial flow turbomachinery.Internal duct losses, which contribute to inefficiency, are proportionalto the square of the velocity of the fluid employed in the operation ofthe turbomachine. It is the purpose of a diffuser to receive therelatively high velocity fluid from the impeller and convert itsvelocity pressure head to static pressure head. The static pressurerecovery coefficient Cp is commonly used to measure effectiveness ofthis conversion process. The parameter Cp is defined as the ratio ofstatic pressure rise to inlet velocity head.

The simplest way to convert velocity pressure head to static pressure ina radial flow turbomachine is by a vaneless diffuser. A diffuser of thistype is merely an open annular area radially displaced about the outerperiphery of the compressor impeller. This arrangement permits the fluidto flow in a natural spiral path from the impeller blade tip to theouter circumference of the vaneless diffuser. However, the circuitousflow path characteristics of the vaneless diffuser arrangement isrelatively long, and therefore subject to generally higher frictionlosses which will reduce the static pressure recovery, Cp.

To overcome this disadvantage inherent in vaneless diffusers, variousattempts have been made to construct flowdirecting passages in thediffuser annulus. It is the purpose of such passages to guide fluid fromthe impeller tip to the outer circumference of the diffuser by a shorterpath than permitted by the vaneless arrangement and thereby reduce theaforementioned friction losses associated with the longer flow path.These passages have heretofore been formed by a row of vanes, eithercurved or straight, or by vane islands. An example of a diffuseremploying curved vanes may be seen in US. Pat. No. 2,819,837 to W. A.Loeb.

' However, each of these arrangements has disadvantages which limitrealization of the greater static pressure recovery permitted by thepresent invention.

Another parameter of significant importance in such vaned diffusers isthe Area Ratio for a given passage which is defined as the ratio of thearea (height times width) at the passage exit to a similar area at thepassage inlet. For a normal divergent passage the Area Ratio will begreater than unity. Also, it is generally accepted that for a givenpassage length to inlet width ratio, there is a finite Area Ratio whichwill yield an optimum Cp. Unfortunately, for single vane rows the AreaRatio required to accomplish the desired reduction in fluid velocity isgreater than the Area Ratio which would yield the optimum Cp.Furthermore, vane islands which attempt to establish an Area Ratio whichwill yield a favorable Cp reduce the amount of velocity pressure headconversion possible for a diffuser of a given radius due to blockage bythe annular volume displaced by the islands.

Therefore, a need still exists for a diffuser that will permit optimumstatic pressure recovery Cp while allowing substantial design freedom asto both reduction of the fluid velocity and overall size of the diffuseren velope.

SUMMARY OF THE INVENTION The present invention has been found to providesuch an advance over the prior diffuser art by employing first andsecond vaned stages in a diffuser, each of the stages comprising aplurality of thin, straight vanes. Various embodiments of such adiffuser are described below and are illustrated in the accompanyingdrawings.

Use of thin, straight vanes in multiple rows to form a cascade has beendetermined as a particularly important feature contributing to a highrate of pressure recovery in a diffuser constructed according to thepresent invention.

Additional features of the preferred embodiments described below furthercontribute to the pressure recovery rate and design versatility of thepresent invention. The trailing edges of the first stage vanes arepreferably tapered to reduce mixing losses for fluid flow between thevaned stages of the diffuser. A vaneless region between the impeller andfirst vaned stage has been found effective for initially conditioningfluid flow from the impeller prior to its entering the first vanedstage. Further, the leading edges of the first stage vanes arepreferably tapered to coincide with the flow direction providing atransition zone for fluid entering the diffuser passages formed betweenthe first stage vanes.

Because of the radially offset arrangement of the vanes, each adjacentpair of vanes provides for a single downstream divergence in the passageformed therebetween. The diffuser walls forming a flow path between theimpeller and collector provide support for the vanes. The walls aregenerally parallel but may also diverge slightly with relation to eachother in a downstream direction providing double divergence in thediffuser passages which may further contribute to the pressure recoveryrate.

The present invention has also been found to contribute to increaseddesign freedom by making possible a diffuser with vanes arranged intandem rows, wherein each row is constructed with an Area Ratio thatwill yield the optimum Cp. The successive blade rows, or cascades, serveto reduce the fluid velocity to a desirable rate.

Additional features of the present invention contributing to unique andadvantageous characteristics of a cascade diffuser having thin, straightvanes are set forth in the following description of various embodimentshaving reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Referring now to the drawings FIG. 1is a sectioned view of a turbomachine including an impeller, a diffuserconstructed according to the present invention and collector means;

FIG. 2 is a fragmentary section view similar to FIG. 1 and more clearlyillustrating the diffuser;

FIG. 3 is a fragmentary view in section taken along one side of thediffuser flow path as seen for example in FIGS. 1 and 2;

FIG. 4 is a view taken along section lines IV -IV of FIG. 3;

FIGS. 5, 6 and 7 illustrate pressure velocity profiles at indicatedlocations in the turbomachine of FIG. 3;

FIGS. 8, 9 and 10 are fragmentary sectioned views similar to FIG. 3illustrating respectively three additional embodiments of a diffuserconstructed according to the present invention; and

FIG. 11 is a fragmentary view similar to FIG. 2 and illustrating adiffuser flow path formed by diverging walls.

DETAILED DESCRIPTION OF THE INVENTION The turbomachine illustrated inFIGS. 1 and 2 includes a housing 12, a shaft 13 driven by means notshown to rotate an impeller 14 having blades 15. Fluid to be compressedenters the housing axially of the shaft from the left as shown inFIG. 1. The fluid is compressed by the impeller and directed radiallyoutwardly at its periphery through an annular diffuser l6 including apair of axially spaced annular walls 17 and 18 forming a flow path 19through the diffuser. Fluid exits the diffuser into a collector 21.

The present invention is directed toward construction of the diffuserwithin the above combination and more particularly to a cascade vaneddiffuser adapted for radial flow. All of the figures illustrate adiffuser having two vaned stages and other features as an optimumarrangement for selected impeller exit conditions. However, a diffuserconstructed according to the present invention may have a varying numberof such stages depending, for example, on engine cycle requirementsestablishing the compressor conditions.

The diffuser 16, illustrated in greater detail in FIGS. 3 and 4,includes a first vaned stage 22 formed by vanes 23 and a second vanedstage 24 formed by vanes 26. The vanes 23 and 26 extend between thewalls 17 and 18. The first stage vanes 23 are arranged in radiallyoffset relation at a selected angle of incidence with relation to theimpeller to form diffuser passages 27 between the adjacent pairs ofvanes. It may be noted that the centerlines of the vanes 23 are disposedalong or close to a vector line representing the direction in whichfluid leaves the impeller and impinges the first stage vanes. Thisarrangement prevents or minimizes undesirable shock waves which mightotherwise develop from interaction of the high velocity gas flow withthe vanes. It is further noted that each adjacent pair of first stagevanes diverges uniformly in a downstream direction at an angledetermined by the number of vanes in the first stage. In the illustrateddiffuser, the first stage includes 32 vanes 23.

The second stage vanes 26 are arranged between the first stage vanes 23to have substantially the same angle of divergence therebetween and tosimilarly form second stage diffuser passages 28.

As noted above, a particularly important feature of the present diffusercontributing to high pressure recovery is the thin, straightconfiguration of each of the vanes 23 and 26. Referring particularly toFIG. 3, each of the vanes has a substantially constant cross-section, atleast in width, except for the leading and trailing edges 31, 32 of thefirst stage vanes 23 and the leading edges 33 of the second stage vanes26 which are tapered for reasons discussed below. It may be noted that,in the embodiment of FIG. 11, the height of the vanes varies somewhatbecause of the slight divergence in the diffuser walls.

The leading edges of the first stage vanes are tapered, preferably bybeveling, to form pressure surfaces 34, each terminating generallyopposite the leading edge 31 of an adjacent first stage vane 23 to forma throat indicated at B in FIG. 3. The throat B, together with thetapered or beveled pressure surface 34, forms a semivaneless region orzone of rapid adjustment for conditioning fluid flow prior to its entryinto the first stage diffuser passages 27.

To minimize a wake effect tending to be caused by the first stage vanesin fluid flow from the first stage vanes to the second stage,'thetrailing edges 32 of the first stage vanes are tapered or beveled as maybe best seen in FIG. 3. The leading edges of the second stage vanes arealso tapered to reduce inlet flow blockage.

To further enhance pressure recovery in the diffuser, the leading edges31 of the first stage vanes are radially spaced apart from the peripheryof the impeller to form a vaneless region annularly arranged around theimpeller and indicated at 36. The vaneless region 36 and thesemi-vaneless region formed by the pressure surfaces 34 have two primaryobjectives of mixing fluid flow from the impeller to obtain a moreuniform velocity profile and reducing the Mach number of the fluid asmuch as possible prior to its entering the passages of the first vanestage. The two vane stages then provide for additional conversion of thefluid velocity pressure head to a static pressure head.

The conversion effect of the diffuser is illustrated by the velocityprofiles illustrated in FIGS. 5-7 representative of different locationsin the turbomachine. FIG. 5 illustrates the uneven velocity profile atthe periphery of the impeller, taken along line A-A of FIG. 3, andindicates the need for uniformity or mixing in the fluid. FIG. 6illustrates the velocity profile at the throat B and shows the effect ofthe vaneless and semi-vaneless regions in accomplishing their objectivesstated above. FIG. 7 illustrates the velocity profile in one of thesecond stage diffuser passages downstream of the first stage vanes, atline C-C.

In the turbomachine of FIG. 3, the impeller has a design speed forexample 51,900 rpm, developing a fluid velocity at its periphery ofalmost sonic speed, for example Mach 0.973. The fluid velocity isreduced through the various portions of the diffuser in the man- I nerdiscussed herein to obtain optimum conversion of Variations possible inthe diffuser vaned stages are illustrated in FIGS. 8-10. Features of thediffuser variations in these figures are indicated by primed numeralscorresponding to those employed in FIGS. 1-4.

In FIGS. 8 and 9, fluid exits straight off the second stage diffuserpassages rather than through a turn as in FIGS. l-4 and 10.

The second stage vanes 26' in FIG. 8 are similarly arranged withrelation to the first stage vanes 23' and each other as in FIG. 3.

However, in FIGS. 9 and 10, an equal number of vanes are employed inboth the first and second stages. The trailing edges of the first stagevanes 23' and the leading edges of the second stage vanes 26' aresimilarly beveled as in FIGS. 3 and 8. Unlike the other embodiments, theleading edges of the second stage vanes 26' are disposed substantiallyupstream from the trailing edges of the first stage vanes 23. Theoverlapping relation of the first and second stage vanes tends tofurther minimize wake effects at the trailing edges of the first stagevanes. In addition, the overlapping vane arrangement illustrates acapability of effectively increasing the length to width ratio for thediffuser passages without increasing the overall diameter of thediffuser.

An additional variation of the present invention is illustrated in FIG.11. The vane arrangement of the two diffuser stages 22 and 24' may besimilar to any of the preceding figures. As noted above, the divergingarrangement of the thin, straight vanes provides single divergence ineach of the diffuser passages. However, in FIG. 11, the diffuser walls17' and 18' diverge slightly in a downstream direction to provide doubledivergence in the passages of either or both diffuser stages. It hasbeen determined that, under various diffuser operating conditions, thedouble divergent configuration of the passages contributes to increasedpressure recovery in the diffuser.

I claim:

1. A cascade diffuser for providing high pressure recovery in radialflow turbomachinery having an impeller and a collector, the diffuserincluding generally parallel spacedapart walls normal to the axis ofsaid impeller providing a flow path between the impeller and collector,the diffuser comprising first and second stage regions, the first andsecond regions each being formed by a plurality of thin vanes ofconstant cross-section along to length extending between the walls andbeing radially offset and diverging slightly with respect to each otherin a downstream direction through at least a portion of the flow pathwith relation to the impeller to form diffuser passages between adjacentones of the vanes which diverge relative to each other in a downstreamdirection toward the collector, each of the vanes being straight andhaving a leading edge and trailing edge, the leading edges of the firststage region vanes being spaced apart in radial relation from theimpeller to provide an initial vaneless stage formed by the diffuserwalls for conditioning fluid flow from the impeller prior to its entryinto the first diffuser stage and intersecting fluid flow from theimpeller with the leading edge of each first stage vane being tapered toprovide a beveled pressure surface extending in a downstream directionand terminating generally'opposite the leading edge of an adjacent firststage vane to provide a throat for the respective diffuser path throughwhich fluid from the impeller passes to the respective diffuser pathsand the trailing edges of the first stage vanes being tapered to reducemixing losses for fluid flow between the first and second stages and theleading edges of the second stage vanes being tapered and locatedgenerally adjacent the trailing edges of the first stage vanes to reduceinlet flow blockage and being arranged in pairs between adjacent ones ofthe first stage vanes an axial opening in one of said parallel wallsbetween each pair of said second region vanes at the radial outer mostportion thereof providing communication between said diffuser and saidcollector.

1. A cascade diffuser for providing high pressure recovery in radialflow turbomachinery having an impeller and a collector, the diffuserincluding generally parallel spacedapart walls normal to the axis ofsaid impeller providing a flow path between the impeller and coLlector,the diffuser comprising first and second stage regions, the first andsecond regions each being formed by a plurality of thin vanes ofconstant cross-section along to length extending between the walls andbeing radially offset and diverging slightly with respect to each otherin a downstream direction through at least a portion of the flow pathwith relation to the impeller to form diffuser passages between adjacentones of the vanes which diverge relative to each other in a downstreamdirection toward the collector, each of the vanes being straight andhaving a leading edge and trailing edge, the leading edges of the firststage region vanes being spaced apart in radial relation from theimpeller to provide an initial vaneless stage formed by the diffuserwalls for conditioning fluid flow from the impeller prior to its entryinto the first diffuser stage and intersecting fluid flow from theimpeller with the leading edge of each first stage vane being tapered toprovide a beveled pressure surface extending in a downstream directionand terminating generally opposite the leading edge of an adjacent firststage vane to provide a throat for the respective diffuser path throughwhich fluid from the impeller passes to the respective diffuser pathsand the trailing edges of the first stage vanes being tapered to reducemixing losses for fluid flow between the first and second stages and theleading edges of the second stage vanes being tapered and locatedgenerally adjacent the trailing edges of the first stage vanes to reduceinlet flow blockage and being arranged in pairs between adjacent ones ofthe first stage vanes an axial opening in one of said parallel wallsbetween each pair of said second region vanes at the radial outer mostportion thereof providing communication between said diffuser and saidcollector.